Logical Appeals

Different Lines of Reasoning

Apply two different lines of reasoning—inductive and deductive—to consciously make sense of observations and reason with the audience.

Learning Objectives

Demonstrate the use of inductive and deductive reasoning

Key Takeaways

Key Points

  • Inductive reasoning, also known as induction, is a kind of reasoning that constructs general propositions that are derived from specific examples.
  • Inductive reasoning is probabilistic; it only states that, given the premises, the conclusion is probable.
  • One important aspect of inductive reasoning is associative reasoning: seeing or noticing similarity among the different events or objects that you observe.
  • Deductive reasoning is the process of reasoning from one or more general statements, laws, or principles regarding what is known, in order to reach a logically certain conclusion.

Key Terms

  • syllogism: An inference in which one proposition (the conclusion) follows necessarily from two other propositions, known as the premises.
  • inductive reasoning: A kind of reasoning that constructs or evaluates general propositions that are derived from specific examples. Inductive reasoning contrasts with deductive reasoning, in which specific examples are derived from general propositions.
  • deductive reasoning: The process of reasoning that uses given true premises to reach a conclusion that is also true. Deductive reasoning contrasts with inductive reasoning.

Reason is the capacity for consciously making sense of things. Reason, or “reasoning,” is associated with thinking, cognition, and intelligence. It is the means by which rational beings understand cause and effect, truth and falsehood, validity, and what is good or bad. The result is a reason that could then be used to explain or justify some event, phenomenon, or behavior.

As you develop arguments for your persuasive speech, you are likely to engage in two different lines of reasoning: inductive, which uses associations, and deductive.

Inductive Reasoning and Associative Reasoning

Inductive reasoning, also known as induction, is a kind of reasoning that constructs general propositions that are derived from specific examples based on previous observations. One important aspect of inductive reasoning is associative reasoning: seeing or noticing similarity among the different events or objects that you observe. For example, if you throw a ball in the air and it comes back down, again and again, you observe the same event happening and are likely to conclude that when you kick a ball in the air, it will come back down.

Inductive reasoning is probabilistic; it only states that, given the premises, the conclusion is probable. Consider these simple logical statements, known as syllogisms. Here is a statistical syllogism to illustrate inductive reasoning:

  1. 90% of humans are right-handed.
  2. Joe is a human.
  3. Therefore, the probability that Joe is right-handed is 90%. If you were required to guess, you would choose “right-handed” in the absence of any other evidence.

Here is another stronger example:

100% of life forms that we know of depend on liquid water to exist. Therefore, if you discover a new life form, it will probably depend on liquid water to exist.

This argument could have been made every time a new life form was found, and would have been correct every time. While it is possible that in the future a life form that does not require water will be discovered, in the absence of other factors, the conclusion is probably correct, as it has been in the past.

Inductive reasoning is used to determine properties or relationships based on previous observations or experiences, and then to formulate general statements or laws based on limited observations of recurring phenomenal patterns. The conclusion of an inductive argument follows with some degree of probability.

Inductive reasoning involves association or analogical reasoning. In order to engage in inductive reasoning, we must observe, see similarities, and make associationsbetween conceptual entities. The ability to structure our perceptions relies on the associative network in our brain, which allows us see the likeness and form a concept, about the similarities.

A chart that shows the differences between deductive and inductive reasoning based on the premise, conclusion, validity, and usage.

Deductive and Inductive Reasoning: There are key differences between deductive and inductive reasoning.

Deductive Reasoning

Deductive reasoning, also called deductive logic, is the process of reasoning from one or more general statements, laws, or principles regarding what is known, in order to reach a logically certain conclusion. Deductive reasoning involves using given, true premises to reach a conclusion that is also true. If you accept or know the general principle as true, then you can apply it to the specific case to conclude that it is also true.

Consider the general principle of the law a gravity: what goes up must come down. Now, when you throw the ball in the air, you conclude that it will fall down based on your knowledge of the general law of gravity. Deductive reasoning contrasts with inductive reasoning in that a specific conclusion is arrived at from the general principle when reasoning deductively. If the rules and logic of deduction are followed, this procedure ensures an accurate conclusion. Here is a classic example of a deductive argument:

  1. All men are mortal.
  2. John is a man.
  3. Therefore, John is mortal.

The first premise states that all objects classified as “men” have the attribute “mortal. ” The second premise states that “John” is classified as a “man”—a member of the class or group of “men. ” The conclusion then states that “John” must be “mortal” because he inherits this attribute from his classification as a “man. ” If both premises are true, the terms are clear, and the rules of deductive logic are followed, then the conclusion of the argument follows by logical necessity.

Conclusion

In summary, with inductive reasoning, you are making observations of specific or particular events and then drawing a general conclusion; whereas with deductive reasoning, you are starting with a general statement and applying it to particular instances when you draw your final conclusion about a particular instance, person, or object.

Deploying a Rational Appeal

A rational appeal uses logical arguments and factual evidence to persuade individuals.

Learning Objectives

Explain how to construct a rational appeal

Key Takeaways

Key Points

  • Deploying rational appeals focuses on reasoning and how you use evidence to reason with your audience and invention, how arguments are formed based on the classical proof of logos –rational appeal and logic.
  • The burden of proof is on you the speaker to develop the right appeals for the particular audience.
  • When deploying deductive reasoning consider whether or not the audience is likely to accept the general premise as valid and true before you attempt to deduce other ideas or courses of action based on the general premise.
  • When deploying inductive reasoning consider if you have observed or collected enough evidence to draw a highly probable conclusion.
  • When deploying associative reasoning, you will want to make sure that the ideas are indeed similar and that there are not obvious or outstanding differences which would negate the association you propose.
  • As you deploy a rational appeal consider if your reasoning is sound, whether the audience will accept your evidence and reasoning, and what objections the audience might raise so you can address the most significant points of disagreement in your message.

Key Terms

  • Invention: the formulation of arguments based on logos–rational appeal or logic.

Forming a Rational Appeal

A rational appeal uses logical arguments and factual evidence to persuade individuals that whatever thesis you are supporting is viable and likely to result in the obtainment of goals. When you focus on rational appeals you are dealing with the mind and cognition of the audience.

A neon sign that has the word "mind" in an oval.

The Mind: When you focus on rational appeals, you are dealing with the audience’s mind and cognition.

The study of rhetoric has historically focused on three types of persuasive appeals– ethos , pathos , and logos. Our focus on reasoning and how you to use evidence to reason with your audience is part of the study of logos. Additionally, you are concerned with invention, which is the first of the five canons of rhetoric identified by Cicero, the classical Roman rhetorician. Invention is how you formulate arguments based on logos–rational appeal or logic.

When you appeal to reason you use logically constructed arguments using your evidence to persuade your audience to agree with you. You might use many different forms of evidence to support your rational appeal. Basically, the burden of proof is on you the speaker as you develop your appeals to the audience. As you deploy a rational appeal consider: Is my reasoning sound, and what will the audience accept as a believable evidence?

Is Your Reasoning Sound?

Prior to your speech, it is important to consider the soundness of your evidence and reasoning.

Deductive reasoning: For example, if you are engaging in deductive reasoning, you will want to consider whether or not the audience is likely to accept the general premise as valid and true before you attempt to deduce other ideas or courses of action based on the general premise.

If you are quoting an authority and drawing conclusions from the authority, it is important to ask if the audience will accept or believe the authority. Remember to quote or use sources that the audience is familiar and will believe; using other authorities or sources will likely not be productive.

Inductive reasoning: If you are engaging in inductive reasoning, you will want to consider whether you have observed or collected enough evidence to draw a highly probable conclusion. Or, did you draw a hasty conclusion based on too few examples or observations?

If you are using statistical evidence as part of your inductive reasoning, it is important to consider how the data was collected and whether it is truly valid. If you do not have valid statistical data, then the inductions will not be valid. Before using any data, ask:

  • Is the source biased, or perceived as biased?
  • Is the source competent in the field being consulted?
  • Is the information current?

Associative reasoning (analogy): When engaging in associative reasoning, you will want to make sure that the ideas are indeed similar and that there are no obvious or outstanding differences which would negate the association in the mind of your audience.

Address Resistance and Concerns of the Audience

Finally, in persuasive situations it is important to anticipate the potential resistance and counterarguments your audience might feel. When you have a sense of what objections the audience might raise, you can and should address the most significant points of disagreement in your message.

Arguments from reason (logical arguments) have some advantages, namely that data are (ostensibly) difficult to manipulate, so it is harder to argue against such an argument; and such arguments make the speaker look prepared and knowledgeable to the audience, enhancing ethos. In addition, if you have built ethos with the audience then it will enhance your appeal with arguments from reason.

Logical Fallacies

A fallacy is an error in reasoning; there are two basic categories of fallacies–formal and informal.

Learning Objectives

Give examples of formal and informal logical fallacies

Key Takeaways

Key Points

  • “Formal” refers to the form of the argument. An argument that contains a formal fallacy will always be invalid. However, even if a premise is not accurate, the formal conclusion could still be valid if the rules of logic are followed.
  • An informal fallacy is an error in reasoning that occurs due to a problem with the content, rather than mere structure, of the argument and is often due to a misconception or a presumption.
  • Common Fallacy–hasty generalization: argues from limited examples or a special case to a create general rule that applies to many cases.
  • Common Fallacy–Popular sentiment or bandwagon appeal (argumentum ad populum):—appeal to the majority; appeal to loyalty, “Everyone is doing it”.
  • Common Fallacy–If it comes before, it is the cause, post hoc ergo propter hoc: believing that temporal succession implies a causal relation.
  • Common Fallacy–Two events co-occurring is not causation, cum hoc ergo propter hoc: believing that correlation implies a causal relation.

Key Terms

  • fallacy: An error in reasoning often due to a misconception or a presumption; used in informal discourse to mean an argument which is problematic for any reason
  • straw man: An insubstantial concept, idea, endeavor or argument, particularly one deliberately set up to be weakly supported, so that it can be easily knocked down; especially to impugn the strength of any related thing or idea.
  • red herring: A clue or information that is or is intended to be misleading, that diverts attention from a question;often thought to relate to using smelly fish to train dogs to recognize the real scent of something they were suppose to be tracking.

Errors in reasoning–formal and informal

A fallacy is an error in reasoning. There are two basic categories of fallacies–formal and informal.

Formal Fallacies

Formal fallacies occur when there is a problem with the form, or structure, of the argument. “Formal” refers to the form of the argument. An argument that contains a formal fallacy will always be invalid.

Consider an example with a visualization of faulty reasoning involving categorical deduction.

An example of a deduction that says "All flowers are animals. All animals can jump. Therefore, all flowers can jump."

Categorical Deduction: Deductive reasoning can be valid, while the major premise is not valid.

  1. All flowers are animals.
  2. All animals can jump.
  3. Therefore, all flowers can jump.

Even though it is quite obvious that the first premise is not true and further that the conclusion is not true, the whole syllogism is still valid. By applying formal logic to the syllogism in the example, the conclusion is still valid.

Informal Fallacies

An informal fallacy is an error in reasoning that occurs due to a problem with the content, rather than mere structure, of the argument. In informal logic and rhetoric, a fallacy is usually an error in reasoning often due to a misconception or a presumption. Some of the more frequent common logical fallacies are:

  • Converse fallacy of accidental or hasty generalization: argues from limited examples or a special case to a general rule. Argument: Every person I’ve met has ten fingers, therefore, all people have ten fingers. Problem: Those, who have been met.are not a representative subset of the entire set.
  • Making the argument personal (argumentum ad hominem): attacking or discrediting the opposition’s character. Argument: What do you know about the U.S? You aren’t even a citizen. Problem: personal argument against an opponent, instead of against the opponent’s argument.
  • Popular sentiment or bandwagon appeal (argumentum ad populum): an appeal to the majority; appeal to loyalty. Argument: Everyone is doing it. Problem: Concludes a proposition to be true because many or most people believe it.
  • Red herring (Ignoratio Elenchi): intentionally or unintentionally misleading or distracting from the actual issue. Argument: I think that we should make the academic requirements stricter for students. I recommend that you support this because we are in a budget crisis and we do not want our salaries affected. Problem: Here the second sentence, though used to support the first, does not address the topic of the first sentence, instead switching the focus to the quite different topic.
  • Fallacy of false cause (non sequitur): incorrectly assumes one thing is the cause of another. Non Sequitur is Latin for “It does not follow. ” Argument: I hear the rain falling outside my window; therefore, the sun is not shining. Problem: The conclusion is false because the sun can shine while it is raining.
  • If it comes before it is the cause (post hoc ergo propter hoc): believing that temporal succession implies a causal relation. Argument: It rained just before the car died. The rain caused the car to break down. Problem: There may be no connection between the two events.
  • Two events co-occurring is not causation (cum hoc ergo propter hoc): believing that correlation implies a causal relation. Argument: More cows die in the summer. More ice cream is consumed in summer months. Therefore, the consumption of ice cream in the summer is killing cows. Problem: No premise suggests the ice cream consumption is causing the deaths. The deaths and consumption could be unrelated, or something else could be causing both, such as summer heat.
  • Fallacy of many questions or loaded question (Plurium Interrogationum): groups more than one question in the form of a single question. Argument: Have you stopped beating your wife? Problem: Either a yes or no answer is an admission of guilt to beating your wife.
  • Straw man: creates the illusion of having refuted a proposition by replacing it with a superficially similar proposition (the “straw man”), and refuting it, without ever actually refuting the original. Argument: Person A: Sunny days are good Person B: If all days were sunny, we’d never have rain, and without rain, we’d have famine and death. Therefore, you are wrong. Problem: B has misrepresented A’s claim by falsely suggesting that A claimed that only sunny days are good, and then B refuted the misrepresented version of the claim, rather than refuting A’s original assertion.
  • The false dilemma or either-or fallacy: the listener is forced to make a choice between two things which are not really related or relevant. Argument: If you are not with us, you are against us. Problem: The presentation of a false choice often reflects a deliberate attempt to eliminate any middle ground.
  • Card-stacking, or cherry picking: deliberate action is taken to bias an argument by selective use of facts with opposing evidence being buried or discredited. Argument: Learn new skills, become a leader and see the world. Problem: Only the positive benefits of military service are used to recruit, and not the hazards.

As a speaker you want to carefully consider your reasoning and how you draw your logical conclusions in order to avoid faulty reasoning.